The 650-kDa 12(S)-Hydroxyeicosatetraenoic Acid Binding Complex: Occurrence in Human Platelets, Identification of Hsp90 as a Constituent, and Binding Properties of Its 50-kDa Subunit

A cytosolic 650-kDa complex which binds 12(S)-hydroxy-5,8,10,14-eicosatetraenoic acid (12(S)-HETE) with high affinity and specificity has been found in various cell lines but not until now in platelet cytosol. After incubation of human platelets with 12(S)-[3H]HETE, a labeled cytosolic 650-kDa complex was isolated. As previously shown for the binding complex in Lewis lung carcinoma (LLC) cells, ATP treatment transformed the platelet complex into a 50-kDa ligand-binding subunit. These results are of interest for two reasons: (a) 12(S)-HETE is a major arachidonic acid metabolite in platelets, and (b) platelets contain large amounts of the cell adhesion molecule GpIIb/IIIa, the activation of which is regulated by 12(S)-HETE. Hsp90 was found to be a component of the 12(S)-HETE binding complex in Lewis lung carcinoma cells, and the 50-kDa ligand-binding subunit itself bound 12(S)-HETE with high affinity. Competition experiments showed that 12(R)-HETE, 15-deoxy-Δ12,14-prostaglandin J2, and 5(S)-HETE had lower affinity for the 50-kDa subunit than 12(S)-HETE. The 12(S)-HETE binding protein appears to be distinct from known members of the steroid hormone receptor superfamily of nuclear receptors.